Pancreatic ?-cells have long been thought to simply be modulators of glucose homeostasis. However, several recent studies have shown an important role for glucagon signaling in regulating pancreatic endocrine cell neogenesis, both in the embryo and from ductal progenitor cells in the adult pancreas. Inhibition of glucagon signaling in the embryonic pancreas, either in vitro or in vivo, will inhibit insulin-positive cell differentiation. We hypothesize that glucagon in the embryonic pancreas is an important regulator of the differentiation of other endocrine lineages, particularly insulin cells. We now have preliminary evidence showing that in utero embryonic treatment with a GLP-1 agonist exendin-4 or a transformed alpha cell line could significantly enhance endocrine differentiation, possibly through different mechanisms depending on the age of the embryo when the treatment was applied. Interestingly, exendin-4 treated mice developed diabetes and obesity but the severity of that depends on the age when the embryonic treatment was given. In this proposal we wish to study the influence of glucagon signaling on the development of other islet cell types, and on pancreatic progenitor cells during normal pancreas development. Because of the key role that glucagon signaling seems to play in the recruitment of new endocrine cells, in this proposal we will characterize the expression and localization of the receptors for glucagon and GLP1 as a potential marker of pancreatic endocrine progenitor cells. Next, we will examine how the glucagon family of peptides regulates ?-cell neogenesis and proliferation during development. The studies in this proposal will provide new insights especially through ?- cell neogenesis. These findings may impact diabetes therapy at multiple levels.